Synapses are fundamental to nervous system function and information processing in the normal and pathological brain. They are highly dynamic structures capable of supporting high firing rates and displaying a broad range of plasticity. A detailed picture of the molecular interactions occurring within a synapse is required to understand how synaptic protein dynamics ultimately shape activity in the nervous system in health and disease. In this proposal, we will investigate Munc13 and complexin (CPX), both highly conserved molecules that are crucial for proper synaptic function. Mice lacking complexin die at birth, and loss of a single isoform is associated with profound locomotor, sensory, and behavioral deficits. Complexin expression is altered in a host of psychiatric and neurodegenerative diseases including Huntington's, Parkinson's, and Alzheimer's disease. Although it is well-established that complexin plays a major role in synaptic function, its mode of action is controversial: there is evidence for both facilitatory and inhibitory roles of this proten in the regulation of synaptic transmission. Munc13 is a large synaptic hub protein that coordinates several proteins involved in synaptic vesicle fusion. Little is known about how neuromodulatory pathways control complexin and Munc13 function at the synapse. We propose to study the regulation of complexin and Munc13 action at the synapse in C. elegans using a combination of biochemistry, electrophysiology, genetics, and in vivo dynamic imaging. We have developed innovative methods of investigating protein interactions in vivo by monitoring the dynamics of proteins exchanging between neighboring synapses using photoactivatable GFP. We will deploy these methods in C. elegans as a model system in which to study the protein interactions of complexin and its binding partners in a functional synapse. By mutating either complexin or its binding partners, we have established that affinity changes lead to mobility changes. Similar approaches can be used to examine Munc13 interactions within individual synaptic boutons. We will also elucidate the structure of complexin and Munc13 protein domains that mediate critical membrane interactions using a combination of biochemical and spectroscopic techniques. The experiments proposed here will provide new insights into the connection between G protein-coupled receptor modulatory pathways and the inhibition of fusion by complexin. Moreover, new regulatory roles for Munc13 and CPX as well as their functional and biochemical connections will be explored.